Understanding GPS-GNSS

Michael Buschermohle

Precision Agriculture Specialist

UT Extension

Global Navigation Satellite System (GNSS)

• Global Positioning System (GPS): 24 Satellites (USA)• GLONASS : 18 Satellites (Russia)• Galileo: 2 Satellites (European Union)• Compass: China• QZSS: Japan• IRNSS: India

Satellite Systems

GNSS = GPS + GLONASS + future additional satellite systems

Advantages of GNSS

Advantages

• Increased accuracy and reliability due to tracking more satellites

GPS

GNSS

Need 5 satellites to maintain RTK-fix

How Does GPS Work

Satellites broadcast several pieces of information• Emphemeris (Orbital) data• Timing data

Sources of GPS Error

• Clock Errors• Atmospheric Interference• Ephemeris (Orbital) Variation• Satellite Configuration• Signal Multipath

Sources of GPS Error

Clock Errors

Let’s put some numbers on it

Altitude = 12,500mi. Speed of light = 186,000 mi/secTransmission Time = 0.067 sec

What happens if we are off a little?

0.001 sec * 186,000 mi/sec = 186 mi

Sources of GPS Error

Atmospheric Interference

Precision AgricultureUniversity of Tennessee Extension

Sources of GPS Error

Ephemeris (Orbital) Variation

Sources of GPS Error

Satellite Configuration

Poor (high) DOP Good (low) DOP

Signal Multipath

Sources of GPS Error

Counteracting GPS Errors

• Land-Based Differential Correction• Satellite-Based Differential Correction• Real-Time Kinematic Correction

Differential GPS (DGPS) Corrections

Satellite-Based Differential GPS Correction

WAASOmniStarJD StarFire

Differential GPS Corrections

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Real-Time Kinematic (RTK) GPS Correction

Base StationVirtual Reference StationContinuous Operating Reference Station (CORS)

GPS Accuracy

• Pass-to-Pass Accuracy

– Represents the short-term (<15 min.) relative accuracy of a GPS/GNSS receiver. You can think of this as the accuracy between adjacent, parallel passes made within 15 minutes of one another.

• GPS/GNSS Drift - Year-To-Year Accuracy

– Drift can be defined as GPS/GNSS receiver (guidance system) accuracy over time. Causes of drift are changes in satellite configuration, operating near trees or other obstacles, and satellite data errors.

GPS Accuracy

Courtesy of Fulton, Auburn University

Correction Service

Pass-to-Pass Accuracy

Potential Range of Drift

WAAS ± 6 to 13 inches ± 4.7 ft

Sub-meter ± 6 to 13 inches ± 2.3 ft

Decimeter ± 2 to 4 inches ± 1.7 ft

RTK ± 1 inch ± 1 inch

Correction Service Pass-to-Pass Accuracy+ Potential Range of Drift*

WAAS + 6 to 13 inches + 4.7 ft

JD StarFire 1OmniStar XP(Sub-Meter)

+ 6 to 13 inches + 2.3 ft

JD StarFire 2OmniStar HP(Decimeter)

+ 2 to 4 inches + 1.7 ft

RTK + 1 inch + 1 inch+Based on manufacturer literature*Based on research conducted at Auburn University

GPS Accuracy

Differential Correction Services

Correction Service Provider Operating Fee/yearWASS FAA NoVBS

OmniSTAR

$800/yearXP $1000/yearHP $1,500/yearStarFire1 John Deere NoStarFire2 $800/yearRTK TN Tractor $800 - 1200/yearRTX Trimble $1000/yearVRS Multiple Providers Varies

CORSTDOT $300/yearEarl Dudley $1000/year

GPS Accuracy

WASS Sub-Meter Decimeter RTK

X X X X Mapping

X X X X Soil Sampling

X X X X Scouting

X X X X Yield Monitoring

X X X X Lightbars

X X X X VRA

X X Section Control

X Auto-Guidance

X Land Leveling

Questions